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Mozilla released a Firefox Nightly test build with vertical tabs - gHacks Tech News
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There are several addons that organize the tabs in the sidebar with a vertical, tree-style layout, with nested tabs that can be collapsed, just like a classic folder structure. This is what GreyBeard was referring to earlier in the thread when he said "The tabs are in a tree hierarchy".
Tree Style Tab has been around since 2007; Sidebery is much newer, and IMO looks and performs better.
You are correct, TreestyleTabs was my jam for years, but I have moved over to Sidebery because it performs better and has better support for containers, as well as being considerably more customizable.